|Media College (Chương 1 : Âm Thanh) (7,8)|
|Written by tuyenphuc|
|Thursday, 18 March 2010 10:57|
Chương 7: Màu sắc âm thanh. Noise, Colours & Types.
Noise Colours & Types
Certain noises are described by their colour, for example, the term "white noise" is common in audio production and other situations. Some of these names are official and technical, others have more loose definitions. These terms generally refer to random noise which may contain a bias towards a certain range of frequencies.
Note: Some of these definitions refer to "all frequencies". This is only theoretical — in practice this means "all frequencies in a finite range".Black Noise
Black noise has various definitions — as far as we are aware none of them are official. Some common definitions are listed below:
(1) Silence, no noise at all.
(2) Noise with a 1/fβ spectrum, where β > 2.
(3) Noise which has zero energy at most frequencies but contains occasional random spikes.
(4) The noise created by active noise control systems, designed to cancel existing noises.
(5) Ultrasonic white noise, i.e. white noise which is at a frequencies too high to hear but which can still affect the environment.
Blue noise, AKA azure noise, is similar to pink noise except the power density increases 3 dB per octave as the frequency increases. In technical terms the density is proportional to f (frequency).
Brown noise is a random noise which mimics the signal noise produced by brownian motion. Technically speaking, the spectral density is proportional to 1/f2, which basically means it has more energy at lower frequencies (decreasing by around 6dB per octave).
To the human ear, brown noise is similar to white noise but at a lower frequency. Examples in nature include waves on the beach and some wind noise.
Note: Some people use the term brown noise as a synonym for brown note, a controversial and unproven sound which causes the listener to lose control of their bowels.
Gray noise is a random noise which sounds the same at all frequencies to the human ear. This is not the same as white noise, which has the same energy at all frequencies. Rather, gray noise is subjected to a "psycho acoustic equal loudness curve" which compensates for the bias of the human ear so that it sounds the same at all frequencies.
Green noise is not an officially recognised term. There are several unofficial definitions in use — these appear to be the two most common:
(1) The mid-frequency component of white noise.
(2) "The background noise of the world", a sort of new-age description of ambient noise averaged from several different outdoors locations. Similar in sound to pink noise with an emphasis on the range around 500Hz.
The semi-official definition of orange noise is "a quasi-stationary noise with a finite power spectrum with a finite number of small bands of zero energy dispersed throughout a continuous spectrum." We have not been able to determine where this definition originated but it is commonly used in reference material.
Orange noise relates to musical scales. The bands of zero energy coincide with the notes in the scale. In effect this means that the in-tune notes of a scale are removed, leaving only the out-of-tune frequencies. This creates a clashing, displeasing noise.
Pink noise (AKA 1/f noise or flicker noise) is similar to white noise except that it contains an equal amount of energy in each octave band. To put it technically, the power spectral density is proportional to the reciprocal of the frequency.
Sound engineers use pink noise to test whether a system has a flat frequency response.
Pink noise can be generated by putting white noise through a pinking filter which removes more energy as the frequency increases (approximately 3 dB per octave).
As white noise is anagous to white light (representing all frequencies equally), pink noise is anagous to light which tends towards the lower end of the visible light spectrum (red light).
Purple noise is similar to brown noise except that the power density increases 6 dB per octave as the frequency increases. In technical terms the density is proportional to f2.
Purple noise is also known as also known as violet noise or differentiated white noise.
Red noise has two common definitions:
(1) Another name for brown noise.
(2) An oceanographic term which describes the ambient noise of distant underwater objects.
The "red" name reportedly refers to the loss of higher frequencies and the emphasis on lower frequencies (this is from the white noise / white light analogy). This would apply to either of the above definitions.
Red noise is not nearly as clearly defined as white or pink noise. Some definitions found on the web conflict with each other, for example, some sources define red noise as a synonym for pink noise.
White noise is a random noise that contains an equal amount of energy in all frequency bands.
White noise is the equivalent of white light, in fact this is how it gets it's name. White light is made up of all light frequencies (colours), while white noise is made up of all audio frequencies.
White noise is used in electronic music, either directly as a sound effect or as the basis to create synthesized sounds. For example, many percussion instruments have a high component of white noise.
White noise is also used to mask other sounds. This process takes advantage of the way the human brain works — the brain is able to single out simple frequency ranges but has trouble when too many frequencies are heard at once. When white noise is present, other noises appear diminished.
White noise is available on CDs etc, marketed as a noise reducer or sleeping aid.
Audio Test Tone
Audio test tones are a special class of artificially-created sounds. An example is the sine-wave tone you sometimes hear at the end of a video, or when a television station goes off the air.
There are two things test tones are usually used for:
DTMF (Dual-Tone Multi-Frequency
Phần 8: Microphone.
Microphones are a type of transducer - a device which converts energy from one form to another. Microphones convert acoustical energy (sound waves) into electrical energy (the audio signal).
Different types of microphone have different ways of converting energy but they all share one thing in common: The diaphragm. This is a thin piece of material (such as paper, plastic or aluminium) which vibrates when it is struck by sound waves. In a typical hand-held mic like the one below, the diaphragm is located in the head of the microphone.
When the diaphragm vibrates, it causes other components in the microphone to vibrate. These vibrations are converted into an electrical current which becomes the audio signal.
Note: At the other end of the audio chain, the loudspeaker is also a transducer - it converts the electrical energy back into acoustical energy.of Microphone
There are a number of different types of microphone in common use. The differences can be divided into two areas:
(1) The type of conversion technology they use
This refers to the technical method the mic uses to convert sound into electricity. The most common technologies are dynamic, condenser, ribbon and crystal. Each has advantages and disadvantages, and each is generally more suited to certain types of application. The following pages will provide details.
(2) The type of application they are designed for
Some mics are designed for general use and can be used effectively in many different situations. Others are very specialised and are only really useful for their intended purpose. Characteristics to look for include directional properties, frequency response and impedance (more on these later).
The electrical current generated by a microphone is very small. Referred to as mic level, this signal is typically measured in millivolts. Before it can be used for anything serious the signal needs to be amplified, usually to line level (typically 0.5 -2V). Being a stronger and more robust signal, line level is the standard signal strength used by audio processing equipment and common domestic equipment such as CD players, tape machines, VCRs, etc.
This amplification is achieved in one or more of the following ways:
Dynamic microphones are versatile and ideal for general-purpose use. They use a simple design with few moving parts. They are relatively sturdy and resilient to rough handling. They are also better suited to handling high volume levels, such as from certain musical instruments or amplifiers. They have no internal amplifier and do not require batteries or external power.
As you may recall from your school science, when a magnet is moved near a coil of wire an electrical current is generated in the wire. Using this electromagnet principle, the dynamic microphone uses a wire coil and magnet to create the audio signal.
The diaphragm is attached to the coil. When the diaphragm vibrates in response to incoming sound waves, the coil moves backwards and forwards past the magnet. This creates a current in the coil which is channeled from the microphone along wires. A common configuration is shown below.
Earlier we mentioned that loudspeakers perform the opposite function of microphones by converting electrical energy into sound waves. This is demonstrated perfectly in the dynamic microphone which is basically a loudspeaker in reverse. When you see a cross-section of a speaker you'll see the similarity with the diagram above. If fact, some intercom systems use the speaker as a microphone. You can also demonstrate this effect by plugging a microphone into the headphone output of your stereo, although we don't recommend it!
Dynamics do not usually have the same flat frequency response as condensers. Instead they tend to have tailored frequency responses for particular applications.
Neodymium magnets are more powerful than conventional magnets, meaning that neodymium microphones can be made smaller, with more linear frequency response and higher output level.
Condenser means capacitor, an electronic component which stores energy in the form of an electrostatic field. The term condenser is actually obsolete but has stuck as the name for this type of microphone, which uses a capacitor to convert acoustical energy into electrical energy.
Condenser microphones require power from a battery or external source. The resulting audio signal is stronger signal than that from a dynamic. Condensers also tend to be more sensitive and responsive than dynamics, making them well-suited to capturing subtle nuances in a sound. They are not ideal for high-volume work, as their sensitivity makes them prone to distort.
A capacitor has two plates with a voltage between them. In the condenser mic, one of these plates is made of very light material and acts as the diaphragm. The diaphragm vibrates when struck by sound waves, changing the distance between the two plates and therefore changing the capacitance. Specifically, when the plates are closer together, capacitance increases and a charge current occurs. When the plates are further apart, capacitance decreases and a discharge current occurs.
A voltage is required across the capacitor for this to work. This voltage is supplied either by a battery in the mic or by external phantom power.
The electret condenser mic uses a special type of capacitor which has a permanent voltage built in during manufacture. This is somewhat like a permanent magnet, in that it doesn't require any external power for operation. However good electret condenders mics usually include a pre-amplifier which does still require power.
Other than this difference, you can think of an electret condenser microphone as being the same as a normal condenser.Technical Notes:
Every microphone has a property known as directionality. This describes the microphone's sensitivity to sound from various directions. Some microphones pick up sound equally from all directions, others pick up sound only from one direction or a particular combination of directions. The types of directionality are divided into three main categories:
To help understand a the directional properties of a particular microphone, user manuals and promotional material often include a graphical representation of the microphone's directionality. This graph is called a polar pattern. Some typical examples are shown below.
Uses: Capturing ambient noise; Situations where sound is coming from many directions; Situations where the mic position must remain fixed while the sound source is moving.
Cardioid means "heart-shaped", which is the type of pick-up pattern these mics use. Sound is picked up mostly from the front, but to a lesser extent the sides as well.
Uses: Emphasising sound from the direction the mic is pointed whilst leaving some latitude for mic movement and ambient noise.
This is exaggerated version of the cardioid pattern. It is very directional and eliminates most sound from the sides and rear. Due to the long thin design of hypercardioids, they are often referred to as shotgun microphones.
Uses a figure-of-eight pattern and picks up sound equally from two opposite directions.
Uses: As you can imagine, there aren't a lot of situations which require this polar pattern. One possibility would be an interview with two people facing each other (with the mic between them).
Some microphones allow you to vary the directional characteristics by selecting omni, cardioid or shotgun patterns.
This feature is sometimes found on video camera microphones, with the idea that you can adjust the directionality to suit the angle of zoom, e.g. have a shotgun mic for long zooms. Some models can even automatically follow the lens zoom angle so the directionality changes from cardioid to shotgun as you zoom in.
Although this seems like a good idea (and can sometimes be handy), variable zoom microphones don't perform particularly well and they often make a noise while zooming. Using different mics will usually produce better results.
When dealing with microphones, one consideration which is often misunderstood or overlooked is the microphone's impedance rating. Perhaps this is because impedance isn't a "critical" factor; that is, microphones will still continue to operate whether or not the best impedance rating is used. However, in order to ensure the best quality and most reliable audio, attention should be paid to getting this factor right.
If you want the short answer, here it is: Low impedance is better than high impedance.
If you're interested in understanding more, read on....
Impedance is an electronics term which measures the amount of opposition a device has to an AC current (such as an audio signal). Technically speaking, it is the combined effect of capacitance, inductance, and resistance on a signal.
Impedance is measured in ohms, shown with the Greek Omega symbol Ω or the letter Z. A microphone with the specification 600Ω has an impedance of 600 ohms.
All microphones have a specification referring to their impedance. This spec may be written on the mic itself (perhaps alongside the directional pattern), or you may need to consult the manual or manufacturer's website.
You will often find that mics with a hard-wired cable and 1/4" jack are high impedance, and mics with separate balanced audio cable and XLR connector are low impedance.
There are three general classifications for microphone impedance. Different manufacturers use slightly different guidelines but the classifications are roughly:
Note that some microphones have the ability to select from different impedance ratings.
High impedance microphones are usually quite cheap. Their main disadvantage is that they do not perform well over long distance cables - after about 5 or 10 metres they begin producing poor quality audio (in particular a loss of high frequencies). In any case these mics are not a good choice for serious work. In fact, although not completely reliable, one of the clues to a microphone's overall quality is the impedance rating.
Low impedance microphones are usually the preferred choice.
Microphones aren't the only things with impedance. Other equipment, such as the input of a sound mixer, also has an ohms rating. Again, you may need to consult the appropriate manual or website to find these values. Be aware that what one system calls "low impedance" may not be the same as your low impedance microphone - you really need to see the ohms value to know exactly what you're dealing with.
A low impedance microphone should generally be connected to an input with the same or higher impedance. If a microphone is connected to an input with lower impedance, there will be a loss of signal strength.In some cases you can use a line matching transformer, which will convert a signal to a different impedance for matching to other components.
Frequency response refers to the way a microphone responds to different frequencies. It is a characteristic of all microphones that some frequencies are exaggerated and others are attenuated (reduced). For example, a frequency response which favours high frequencies means that the resulting audio output will sound more trebly than the original sound.
A microphone's frequency response pattern is shown using a chart like the one below and referred to as a frequency response curve. The x axis shows frequency in Hertz, the y axis shows response in decibels. A higher value means that frequency will be exaggerated, a lower value means the frequency is attenuated. In this example, frequencies around 5 - kHz are boosted while frequencies above 10kHz and below 100Hz are attenuated. This is a typical response curve for a vocal microphone.
An ideal "flat" frequency response means that the microphone is equally sensitive to all frequencies. In this case, no frequencies would be exaggerated or reduced (the chart above would show a flat line), resulting in a more accurate representation of the original sound. We therefore say that a flat frequency response produces the purest audio.
In the real world a perfectly flat response is not possible and even the best "flat response" microphones have some deviation.
More importantly, it should be noted that a flat frequency response is not always the most desirable option. In many cases a tailored frequency response is more useful. For example, a response pattern designed to emphasise the frequencies in a human voice would be well suited to picking up speech in an environment with lots of low-frequency background noise.
The main thing is to avoid response patterns which emphasise the wrong frequencies. For example, a vocal mic is a poor choice for picking up the low frequencies of a bass drum.
You will often see frequency response quoted as a range between two figures. This is a simple (or perhaps "simplistic") way to see which frequencies a microphone is capable of capturing effectively. For example, a microphone which is said to have a frequency response of 20 Hz to 20 kHz can reproduce all frequencies within this range. Frequencies outside this range will be reproduced to a much lesser extent or not at all.
This specification makes no mention of the response curve, or how successfully the various frequencies will be reproduced. Like many specifications, it should be taken as a guide only.
Condenser microphones generally have flatter frequency responses than dynamic. All other things being equal, this would usually mean that a condenser is more desirable if accurate sound is a prime consideration.
This tutorial aims to provide you with the skills to choose the correct microphone and use it properly to obtain the best possible sound. It is suitable for people interested in any type of audio or video work. Before you begin you should have a basic understanding of the most common types of microphone and how they work. If you don't, read how microphones work first.
The tutorial is six pages and takes about 20 minutes to complete.
The microphone (mic) is a ubiquitous piece of equipment. Found in everything from telephones to computers to recording studios, microphones are part of our daily life.
Few people think about the microphone in their telephone when they use it. Some people think about the microphone on their video camera when they use it. All professionals pay careful attention to their microphones whenever they use them.
Don't make the mistake that many amateurs make and use whatever mic is at hand (e.g. using a vocal mic for a bass drum). Also, don't make the mistake of assuming that using a microphone is easy. Microphone technique is a learned skill - plugging it in and pointing it isn't always enough.
As we discussed in the previous tutorial, there are many different types of microphone in common use. The differences are usually described in two ways: The technology they use (e.g. dynamic, condenser, etc) and their directionality (e.g. omnidirectional, cardioid, etc). In addition, microphones have a number of other characteristics which need to be taken into account.
When choosing a microphone, the first thing you will need to know is what characteristics you need. After that, you can worry about things like size, brand, cost, etc.
Note: If you haven't done so already, you might like to do some groundwork and read how microphones work first.
Work through each of these characteristics and determine your needs.
Decide which type of directional pattern best fits your needs. Remember that it's usually better to use a less directional mic in a position close to the sound source, than to be further away using a hypercardioid. For more information see microphone directional characteristics.
Make sure the mic's frequency response is appropriate for the intended use. As a rule of thumb flat response patterns are best, but in many cases a tailored response will be even better. For more information see microphone frequency response.
The rule of thumb is: Low impedance is better than high impedance. For more information see microphone impedance.
Remember that the diaphragm works by converting vibrations from sound waves into an electrical signal. Unless the microphone has some sort of protection system, the diapragm can't tell the difference between a desirable sound wave vibration and any other sort of vibration (such as a person tapping the microphone casing). Any sort of vibration at all will become part of the generated audio signal.
If your mic is likely to be subjected to any sort of handling noise or vibration, you will need a mic which will help prevent this noise from being picked up. High quality hand-held mics usually attempt to isolate the diaphragm from vibrations using foam padding, suspension, or some other method. Low quality mics tend to transfer vibrations from the casing right into the diaphragm, resulting in a terrible noise.
Note that lavalier mics don't usually have protection from handling noise, simply because they are too small to incorporate any padding. It is therefore important to make sure they won't be moved or bumped.
If you can afford it, it makes sense to buy a range of microphones and use the most appropriate one for each job. If your budget is more limited, think about all the different things you need to use the mic for and try to find something which will do a reasonable job of as many of them as possible.
In the end, sound is quite subjective. You really want a mic which will provide the sound you like. A good idea is to set up a contolled test. Record the same sounds using different mics, keeping all other factors constant.
Make sure you are comparing apples with apples; for example, don't compare a hand-held cardioid and a shotgun in the same position. If you do want to compare these mics, make sure each is placed in its optimum position.
The golden rule of microphone placement is get the distance right. In general, place the microphone as close as practical to the sound source without getting so close that you introduce unwanted effects (see below).
The aim is to achieve a good balance between the subject sound and the ambient noise. In most cases you want the subject sound to be the clear focus, filled out with a moderate or low level of ambient noise. The desired balance will vary depending on the situation and the required effect. For example, interviews usually work best with very low ambient noise. However if you want to point out to your audience that the surroundings are very noisy you could hold the mic slightly further away from the subject.
It is possible to get too close. Some examples:
When using more than one microphone you need to be wary of phasing, or cancellation. Due to the way sound waves interfere with each other, problems can occur when the same sound source is picked up from different mics placed at slightly different distances. A common example is an interview situation in which two people each have a hand-held mic - when one person talks they are picked up by both mics and the resulting interference creates a phasing effect.
You don't always have to conform to standard ways of doing things. As long as you're not placing a microphone in danger there's no reason not to use them in unusual positions. For example, lavalier mics can be very versatile due to their small size - they can be placed in positions which would be unrealistic for larger mics.
Guitar amps are miced very closely. This helps keep the sound isolated from the rest of the stage noise. Theoretically the amp will not create any level burst strong enough to distort the microphone.
Snare drum mics need to be close to the skin without getting in the way of the drummer or risking damage.
Microphone Stands, Mounts & Clamps
An important consideration is the way the microphone is held or mounted. A poorly mounted mic can lead to all sorts of problems, whereas a well-mounted mic can lift the audio quality significantly. Things to consider when mounting a mic include:
There are many ways to mount microphones. Let's look at the most common methods...
The most obvious mount is the microphone stand. There are three main variations: The straight vertical stand, the boom stand and the small table-top stand.
Boom stands are very useful and versatile. If you are considering buying a general-purpose stand, a boom stand is the logical choice.
Some things to watch out for when setting up a microphone stand:
Note: Boom arms controlled by sound operators will be covered on the next page.
Instead of using a dedicated mic stand, you can use a specialised clamp to piggyback on another stand (or any other object).
Clamps are often used in musical situations where there are many stands and many microphones. The classic example is the drum kit which is surrounded by cymbal stands - clamps are well suited to this application.
Lavalier (lapel or lap) mics are usually attached to the subject's clothing using a specialised clip. Obviously the preferred position is on the lapel or thereabouts. This provides consistent close-range sound pickup and is ideal for interview situations in which each participant has their own mic. It also means the subject doesn't have to worry about mic technique.
If you have time, discreetly hide the cable in the clothing. If there is nowhere to place the mic on the subject's chest, try the collar.
A headset with its own mic works well in situations such as:
Headsets are ideal for stage performers, as well as sports commentators, radio announcers, etc. Like lav mics, they provide very consistent audio.
In order to minimise unwanted noise caused by vibration of the stand or mount, a shock absorption system may be used. This isolates the mic from the vibrations, usually with foam padding or elastic suspension.
Hết chương 1.